1. Field of the Invention
The present invention relates to a method for growing a semiconductor by vapor phase epitaxy, and more particularly to a method for growing a low-resistance p-type gallium nitride (GaN)-related compound semiconductor film by vapor phase epitaxy.
2. Description of the Related Art
Conventional methods for preparing a p-type gallium nitride-based compound semiconductor by vapor phase epitaxy involve use of a magnesium organo-metallic compound as a p-type dopant source material. Magnesium is known to have the lowest acceptor level of presently know p-type impurities for GaN related compound semiconductors.
However, it is known that the crystal immediately after growth has increased resistance due to the so-called "hydrogen passivation" phenomenon. That is, magnesium is electrically inactivated through its combination with hydrogen which is contained in the gases used for vapor phase epitaxy.
FIG. 6 is a schematic diagram illustrative of a substrate temperature variation during a process sequence of a conventional growth of a p-type gallium nitride crystal. Types of gases introduced into the crystal growth chamber (reaction chamber) is also shown in the figure (c.f. S. Nakamura, et al., Japanese Journal of Applied Physics, vol. 30, No. 10A, pp. L1708-L1711, 1991, for example). As shown in FIG. 6, the crystal grown temperature is 1030.degree. C. The substrate is sapphire, the carrier gas used during the growth process is hydrogen, the gallium source material is trimethyl gallium (TMG), the nitrogen source material is ammonia, and the p-type dopant source material is cyclopenta-diethyl magnesium (CP2Mg).
After the crystal has been grown, the cooling of the substrate is performed in an atmosphere of hydrogen carrier gas and ammonia. In the process sequence of FIG. 6, hydrogen is not introduced into the crystal while the crystal is growing at high temperatures. However, hydrogen is diffused into the crystal from the surface during the process of cooling the substrate crystal after the growth has been completed and is combined with magnesium thus passivating magnesium with hydrogen. The source of hydrogen while the substrate crystal is cooled includes hydrogen which is directly bonded with nitrogen of the ammonia molecule and hydrogen of the hydrogen carrier gas.
FIG. 7 illustrates the layer structure of an LED (light-emitting diode) crystal which is prepared by the method of growing a p-type gallium nitride by vapor phase epitaxy as illustrated in FIG. 6 (c.f. S. Nakamura et. al., Applied Physics Letter 64, No. 13, pp. 1687-1689, 1994).
The LED crystal consists of a Gab buffer layer 16, an Si-doped n-type GaN layer 17, an InGaN active layer 18, an Mg-doped p-type AlGaN layer 19 and an Mg-doped p-type GaN layer 20 on a sapphire substrate 15. Methods which have been suggested for decreasing the resistance of a high-resistance p-type gallium nitride-based compound semiconductor which has undergone hydrogen passivation as a result of vapor phase epitaxy include irradiation with low-energy electron beams (see Japanese Unexamined Patent Application Disclosure HEI 3-218625), and thermal annealing in a vapor phase atmosphere free of hydrogen atoms at a temperature 400.degree. C. or higher, preferably approximately 700.degree. C. (Japanese Unexamined Patent Application Disclosure HEI 5-183189).
Furthermore, several methods without the necessity of special treatment after the crystal is grown have been suggested for providing a low-resistance p-type GaN based compound semiconductor film by vapor phase epitaxy, These include growth of magnesium-doped gallium nitride on an In.sub.x Al.sub.y Ga.sub.1-x-y N (0&lt;x&lt;1, 0.ltoreq.y&lt;1) layer (see Japanese Unexamined Patent Application Disclosure HEI 6-232451). This method is aimed at providing a low-resistance p-type layer by using an indium-containing gallium nitride-based compound semiconductor which is relatively soft, thus suppressing defect formation in gallium nitride films which, grow thereon. However, the attainable layer structure is limited and the effect of the resistance decrease is smaller than thermal annealing.
Another known method for obtaining low-resistance p-type gallium nitride is by molecular beam epitaxy in which the growth is accomplished using hydrogen-free source materials only. Here, metal gallium is used as the gallium source material, and nitrogen plasma is used as the nitrogen source material. However, the crystals presently gained by molecular beam epitaxy are, inferior to ones provided by vapor phase epitaxy which is more suitable for mass production.
Regarding treatments after vapor phase epitaxy aimed at lowering the resistance of p-type gallium nitride, irradiation by electron beams has the problem of low productivity, since only layers of thickness matching the penetration depth of the electron beams (on the order of 0.5 .mu.m) have lower resistance.
The other method, thermal annealing, though capable of lowering the resistance of layers having thicknesses as large as several .mu.m, and thus having increased productivity, causes desorption of nitrogen from the gallium nitride, inevitably leading to thermal deterioration of the crystal. The thermal deterioration of the crystal becomes more severe as the annealing is performed at a higher temperature.